The basic component of a tunnel spin injector and a magnetic tunnel junction is a ferromagnetic layer combined with a tunnel barrier. The basic structure of an MTJ is a sandwich of two thin ferromagnetic and/or ferrimagnetic layers separated by a very thin insulating layer. In both the spin injector and the MTJ, the electrons that tunnel from the ferromagnetic electrode across the tunnel barrier are spin polarized. The degree of spin polarization depends on both the composition and nature of the ferromagnetic metal, the tunnel barrier, and the interface between the two. In an MTJ the tunneling current is typically higher when the magnetic moments of the ferromagnetic (F) layers are parallel and lower when the magnetic moments of the two ferromagnetic layers are anti-parallel. The change in conductance for these two magnetic states can be described as a magnetoresistance. Here the tunneling magnetoresistance (TMR) of the MTJ is defined as (RAP−RP)/RP where RP and RAP are the resistance of the MTJ for parallel and anti-parallel alignment of the ferromagnetic layers, respectively. MTJ devices have been proposed as memory cells for nonvolatile solid state memory and as external magnetic field sensors, such as TMR read sensors for heads for magnetic recording systems. For a memory cell application, one of the ferromagnetic layers in the MTJ is the reference layer and has its magnetic moment fixed or pinned, so that its magnetic moment is unaffected by the presence of the magnetic fields applied to the device during its operation. The other ferromagnetic layer in the sandwich is the storage layer, whose moment responds to magnetic fields applied during operation of the device.
In the quiescent state, in the absence of any applied magnetic field within the memory cell, the storage layer magnetic moment is designed to be either parallel (P) or anti-parallel (AP) to the magnetic moment of the reference ferromagnetic layer. For a TMR field sensor for read head applications, the reference ferromagnetic layer has its magnetic moment fixed or pinned so as to be generally perpendicular to the magnetic moment of the free or sensing ferromagnetic layer in the absence of an external magnetic field. The use of an MTJ device as a memory cell in an MRAM array is described in U.S. Pat. No. 5,640,343. The use of an MTJ device as a MR read head has been described in U.S. Pat. Nos. 5,390,061; 5,650,958; 5,729,410 and 5,764,567.
FIG. 1A illustrates a cross-section of a conventional prior-art MTJ device. The MTJ 100 includes a bottom “fixed” or “reference” ferromagnetic (F) layer 15, an insulating tunnel barrier layer 24, and a top “free” or “storage” ferromagnetic layer 34. The MTJ 100 has bottom and top electrical leads 12 and 36, respectively, with the bottom lead being formed on a suitable substrate 11, such as a silicon oxide layer. The ferromagnetic layer 15 is called the fixed (or reference) layer because its magnetic moment is prevented from rotating in the presence of an applied magnetic field in the desired range of interest for the MTJ device, e.g., the magnetic field caused by the write current applied to the memory cell from the read/write circuitry of the MRAM. The magnetic moment of the ferromagnetic layer 15, whose direction is indicated by the arrow 90 in FIG. 1A, can be fixed by forming it from a high coercivity magnetic material or by exchange coupling it to an antiferromagnetic layer 16. The magnetic moment of the free ferromagnetic layer 34 is not fixed, and is thus free to rotate in the presence of an applied magnetic field in the range of interest.
In the absence of an applied magnetic field, the moments of the ferromagnetic layers 15 and 34 are aligned generally parallel (or anti-parallel) in an MTJ memory cell (as indicated by the double-headed arrow 80 in FIG. 1A) and generally perpendicular in a MTJ magnetoresistive read head. The relative orientation of the magnetic moments of the ferromagnetic layers 15, 34 affects the tunneling current and thus the electrical resistance of the MTJ device. The bottom lead 12, the antiferromagnetic layer 16, and the fixed ferromagnetic layer 15 together may be regarded as constituting the lower electrode 10. In FIG. 1B, the reference ferromagnetic layer 15 is replaced by a sandwich of two ferromagnetic layers 18 and 19 antiferromagnetically coupled through a metallic spacer layer 17 as shown by the MTJ 100′ of FIG. 1B. The lower electrode is now given by the reference numeral 10′, and the magnetic orientation of the layers 18 and 19 is given by the arrows 90′ and 95, respectively.
MTJs can display large tunneling magnetoresistance (TMR) at room temperature of up to 70% using Al2O3 tunnel barriers and more than 220% using MgO tunnel barriers (S. S. P. Parkin et al., Nature Materials 3, 862 (2004)). The resistance of the MTJ depends on the relative orientation of the magnetizations of the F electrodes. Here we define TMR=(RAP−RP)/RL where RAP and RP correspond to the resistance for anti-parallel and parallel alignment of the F electrodes' magnetizations, respectively, and RL is the lower of either RP or RAP. The TMR originates from the spin polarization of the tunneling current which can be measured most directly using superconducting tunneling spectroscopy (STS) in related tunnel junctions in which one of the ferromagnetic electrodes of the MTJ is replaced by a thin superconducting (S) layer. The TMR and spin polarization are then simply related according to Julliere's model (M. Juliere, Phys. Lett. 54A, 225 (1975)).
For applications of magnetic tunnel junctions for either magnetic recording heads or for non-volatile magnetic memory storage cells, high TMR values are needed for improving the performance of these devices. The speed of operation of the recording head or memory is related to the signal to noise ratio (SNR) provided by the MTJ—higher TMR values will lead to higher SNR values for otherwise the same resistance. Moreover, for memory applications, the larger the TMR, the greater is the device-to-device variation in resistance of the MTJs that can be tolerated. Since the resistance of an MTJ depends exponentially on the thickness of the tunneling barrier, small variations in thickness can give rise to large changes in the resistance of the MTJ. Thus high TMR values can be used to mitigate inevitable variations in tunnel barrier thickness from device to device. The resistance of an MTJ device increases inversely with the area of the device. As the density of memory devices increases in the future, the thickness of the tunnel barrier will have to be reduced (for otherwise the same tunnel barrier material) to maintain an optimal resistance of the MTJ memory cell for matching to electronic circuits. Thus a given variation in thickness of the tunnel barrier (introduced by whatever process is used to fabricate the MTJ) will become an increasingly larger proportion of the reduced tunnel barrier thickness and so will likely give rise to larger variations in the resistance of the MTJ device.
Different tunnel barrier materials have distinct advantages and disadvantages. For example, MgO tunnel barriers exhibit high tunneling magnetoresistance and tunneling spin polarization, have very high thermal stability and have relatively low resistance for the same tunnel barrier thickness as compared to, for example, aluminum oxide tunnel barriers. A potential disadvantage of crystalline MgO tunnel barriers is that the magnetic properties of the free or sensing magnetic layer, adjacent to the MgO barrier, may be influenced by the crystallinity of the MgO layer, leading possibly to greater variations in magnetic switching fields, from device to device, than are seen using amorphous barriers with no well defined crystallographic structure. However, this can be mitigated by the use of amorphous ferromagnetic electrodes. Another potential disadvantage of both MgO and alumina tunnel barriers is that they have high tunnel barrier heights: The tunnel barrier height is related to the electronic band gap of the insulating material, and the band gaps of MgO and alumina are high. For applications where the device size is deep sub-micron in size and for ultra high speed applications, such as for advanced magnetic recording read head elements, lower tunnel barrier heights may be advantageous since these allow for lower resistance-area products or for thicker tunnel barriers with the same resistance-area product.
What is needed are tunnel barrier materials which give rise to substantial tunneling magnetoresistance at low resistance-area products and, which, when formed on magnetic electrodes, do not substantially oxidize the underlying magnetic electrode, which would otherwise depress the magnitude of the spin polarization of the tunneling current and the tunneling magnetoresistance of magnetic tunnel junctions using these tunnel barriers.